I upgraded my interface from a Stienberg C2 and now use a Focusrite Saffire Pro 24.
The Saffire Pro 24 is Firwire/USB and I am using the Firewire.
Sonar X1 has had much better perforamce, less latency mic imputs are very nice and the interface came with some loops a Focusrite Plus-in suite wich was an added bonus (decent effects plugins free).
Ableton works much much better with this interface and I notice less drop outs, latency issues and it helped CPU hit a bit. not sure if it cause I'm using Firewire or not but the interface made a real difference.
Pro 24 runs about $299 list but I managed to get it with a guitar center coupon for $199 or price matching via another vendor like amazon.com I can't remember - I didn't pay that much and with all the other interfaces I got a real deal for a quality interface.
I am also running Windows 7 64 bit and have a quad core 2.4ghz with 8gb ram and ssd hard drive. all of that plays into performance as well.